In several applications there is a need for emitting electromagnetic radiation containing two different wavelengths towards a specific point. Such an application may be a spectrophotometric analysis, where a sample is irradiated with the two different wavelengths and the interaction of the electromagnetic radiation and the sample is registered. The two different wavelengths are needed, since properties of the sample may be determined by establishing a ratio between the interactions of the sample with the different wavelengths.
There are a number of solutions to the need for emitting electromagnetic radiation containing two wavelengths towards a sample. According to a first solution, the sample is first irradiated by radiation of a first wavelength and then irradiated by radiation of a second wavelength. This may be accomplished by for example selectively activating a mirror for optionally directing the first radiation or the second radiation towards the sample to be irradiated. However, this requires a mechanically moveable component, which sets high demands on the stability of the setup and also makes the setup complex.
According to a second solution, the sample is irradiated by different wavelengths in different positions. However, then the radiation of different wavelengths will not interact with the same parts of the sample. Therefore, differing properties of the different positions of the sample, such as differing thickness of the sample, could affect the result of the analysis.
According to a third solution, the sample may be irradiated by a source which emits a wide spectrum of wavelengths. However, if two specific wavelengths are needed, there may not be any source that emits these two wavelengths.
According to a fourth solution, a beam splitter may be used. Radiation incident on a beam splitter will be partly reflected and partly transmitted. Radiation of a first wavelength may then incide on one side of the beam splitter and radiation of a second wavelength may incide on the other side of the beam splitter. Part of the radiation of the first wavelength will then be transmitted through the beam splitter and will be collimated with the part of the radiation of the second wavelength that is reflected by the beam splitter. However, only 50% of the combined radiation intensity of the wavelengths may be used in the measurement, since the rest of the radiation is directed by the beam splitter-away from the sample position.
According to a fifth solution, an active component may be used. The active component acts in a similar manner as the beam splitter. However, by applying a voltage over the active component, it may reflect 100% of the radiation incident on one side. When the voltage is turned off, the active component transmits 50% of the radiation incident on the other side. The active component requires an extra electric circuit for its activation. This makes the design of an optical setup using the active component more complex. Further, a measurement requires switching on and off the voltage applied to the active component.